This invention generally relates to a device for detecting radiation in the near infrared (IR) spectrum. In particular, the invention relates to a low noise IR detector that operates by transferring charge rather than by charging and resetting a capacitor through which voltage is read.
Modern infrared (IR) imaging systems can be focal plane arrays of detectors and associated integrated circuitry in each pixel that transforms the collected signals into visual or other analyzable forms. Near IR detector systems that operate in the 1 to 1.7 μm wavelength region are sometimes combined with visible detection systems that operate in the 400 to 700 nm wavelength range to enhance detection and visualization in low light and early night scenarios. Combined visible and near IR imaging capability is increasingly becoming a strategic requirement for both commercial and military applications. Of the many materials used for imaging systems that operate in the near infrared (e.g. HgCdTe, Ge, InSb, PtSi, etc.), InGaAs p-i-n photodiodes have been chosen due to their high performance and reliability (G. Olsen, et al., “A 128×128 InGaAs detector array for 1.0-1.7 microns,” in Proceedings SPIE, Vol. 1341, 1990, pp. 432-437).
Short wavelength infrared (SWIR) imaging arrays are normally hybrid devices where the photodiodes are interconnected to silicon transistor read out integrated circuitry (ROIC). In one effort to decrease cost and simplify complex manufacturing, an InGaAs/InP photodiode has been integrated with an InP junction field effect transistor (JFET) as a switching element for each pixel, as described by U.S. Pat. No. 6,005,266, Forrest et al. (which is incorporated herein by reference in its entirety). The combination of photodiode and FET on a single substrate enabled the formation of fully monolithic near IR focal plane arrays with reduced production cost and increased performance. The InP junction field effect transistors exhibited leakage currents as low as 2 pA. In related work, intentional doping of the absorption layer of a GaAs p-i-n photodiode was found to reduce the dark current as described by U.S. Pat. No. 6,573,581, Sugg. et al. (which is incorporated herein by reference in its entirety).
In previous detectors, light induced charge is collected in a single area that is then transferred to an external capacitor where the voltage on the capacitor is measured. The capacitor is then “reset” before the next measurement. Since it is difficult to completely reset a capacitor in a finite amount of time, and the collection area may be collecting charge during the reading operation itself, opportunities exist for variation in the amount of signal read.